With development of technology and people's attention to audio-visual entertainment, the application of digital microphones becomes widespread. The uses of digital microphones are essential no matter in public places (e.g. corporate meeting rooms, public exhibition places) or private places (personal audio-visual rooms). In general, a Microelectromechanical Systems (MEMS) and an Application Specific Integrated Circuit (ASIC) are disposed in an internal circuit of a digital microphone. A source follower, a Programmable Gain Amplifier (PGA) and an Analog to Digital converter (ADC) are used for processing or buffering an electric signal so as to convert the electric signal to a digital signal after the electric signal obtained by the MEMS sensor for achieving a high signal-to-noise ratio (SNR). However, this kind of circuit coming with the source follower, the PGA and the ADC consumes a huge amount of current and occupies a space in a circuit.